This invention relates generally to axial flow fans and, more particularly, to a method and apparatus for reducing their clearance flow losses.
Axial flow fans are used in a wide variety of applications, including HVAC, refrigeration, automotive, power systems and aerospace. In each of these applications, efficiency and space limitations are especially important considerations.
Significant efficiency loss occurs in axial flow fans due to backflow in the clearance region between the fan rotor and the casing. The rotor may utilize conventional blades that extend outward with blade tips approaching the casing, or it may utilize blades that include a rotating shroud attached to the blade tips. In either case backflow is driven from the high pressure side of the rotor to the suction side across the clearance gap, leading to reduced performance, increased noise level and reduced stability and stall-margin.
Various designs have been proposed for increasing fan efficiency by reducing or controlling clearance flows. The designs generally involve an interruption or decrease in the size of the gap. One approach is the use of a tip seal structure wherein a circumferentially extending groove in the casing circumscribes the tips of the blades as shown and described in U.S. Pat. No. 4,238,170. In another approach, an axial fan is provided with a casing having a bellmouth, and the shroud is so formed as to create a separation bubble between the bellmouth and the shroud in order to limit the circulation flow as shown in U.S. Pat. No. 7,086,825 assigned to the assignee of the present invention.
Fan stability is affected by rotating flows within the clearance gap. These flows tend to develop into organized rotating cells which can lead to strong through-flow oscillations and excessive noise.
Various designs have been proposed to improve fan stability by controlling these rotating flows. These designs are generally classified as casing treatment.